The invention provides a compound feed additive which is prepared by compounding hawthorn powder, portulaca oleracea herb micro powder, honeysuckle extract, dandelion extract, grape seed extract, echinacea extract, astragalus extract, common andrographis herb extract, numerous types of plant materials subjected to ultramicro grinding or extracting. The plant extract feed additive is prepared by selecting natural plants through the processes of micro grinding, extracting, concentrating and the like, and has the characteristics of wide raw material sources, high biological utilization rate, no toxicity and no residue, and antibiotics are prevented from being accumulated in the livestock body at the source. The compound feed additive can obviously reduce the piglet diarrhea which is caused by various reasons, the daily increased weight of the piglet is increased, and the humoral immunity and cell immunity functions of the piglet are improved.